Elimination of frequency shift in suppressed carrier systems



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ATTORNEY March 30, 1965 u. s. BERGER ELIMINATION OF FREQUENCY SHIFT IN SUPPRESSED CARRIER SYSTEMS 2 Sheets-Sheet 2 Filed NOV. 28, 1961 Bf Km- A TTORNEY United States Patent O Fried Nov. 2s, 1961, ser. No. 155,381 s ciaims. (ci. S25- 49) This invention relates generally to the vtransmission of signals by amplitude modulation techniques and more particularly to suppressed-carrier amplitude modulation signal transmission over media which tend to introduce uncontrolled frequency shifts.

When either or both of the sidebands of an amplitude modulated wave are transmitted over a medium which introduces a frequency shift, the received signal is likely to be distorted if a carrier wave equal to the original carrier wave in frequency is used to accomplish demodulation. Under such conditions, the sidebands may undergo frequency changes not shared by the carrier and the demodulated signal is likelyto incur not onlyfrequency error but also quality degradation due to misalignment with receiving band filters. In the past, suchdistortions have been avoided-by transmitting the carrier at full or reduced amplitude along with the sidebands, by manual adjustment of the demodulating` carrier to cancel the frequency shift, or by taking special measures to keep the amount of frequency shift introduced by the transmission system within narrow limits.

There are, however, a number of reasons why none of these expedients may be desirable. Transmission of carrier atfull amplitude may, for example, overload repeaters in the transmission system or cause crosstalk between adjacent lines, while transmission of carrier. at

reduced amplitude may` lead to filtering difficulties `in separating the carrier at the receiving end of the system for amplification prior to demodulation. Manual adjustment of the demodulating carrier frequency, on the other hand, requires a separate adjustment for every change in operating conditions. Controlling the` frequency shift introduced by the transmission system within narrow limits, finally, tends to require relatively. compleX and expensive apparatus and is subject to failure in the event of failure of any portion of the Vcontrolling mechanism. i Y

A principal object of the invention is, therefore, to eliminate frequency errors in the transmission of suppressed-carrieramplitude modulated waves without requiring either manual adjustments or measuresto limit the amount of frequency shift in the transmitting medium.,

A closely related object is to eliminate frequency error in the transmission of Vsuppressed-carrier amplitude modulated waves in as simple and reliable a manner as possible.

A typical amplitude modulation transmission `system which'tends occasionally to introduce a small amount of 3,176,226 Patented Mar. 30, 1965 ice known in the art as frequency frogging and tends to equalize noise and crosstalk in the individual signal channels. Any instability or inaccuracy in the locally generated group carrier frequencies, however, will result in a frequency lshift that is cumulative throughout the system. If it reaches an order of magnitude ofV 100 cycles per second, it can result in degradation of channel frequency characteristics because of misalignment of the received frequency spectrum with respect to the receiving channel band filters. In addition, while the frequency shift may be otherwise tolerable for normal transmission of carrier-transmitted channel signals, it may not be acceptable for program and certain types of data signals which are not accompanied by their own carrier and hence require more faithful reproduction of frequency.

In accordance with the invention, the final group demodulating carrier in such a system as the type N carrier system is derived, not from a local oscilla-tor, but rather by frequency multiplication and intermodulation techniques from a pair of pilot frequencies which are within the pass band of the system. When the system is used to transmit groups of double-sideband carrier-transmitted channel signals, these pilot frequencies may be and preferably are two of the individual channel carriers. At the final group demodulator, which is otherwise like the preceding group modulators except that it is the last one in the system, the demodulating carrier derived from these pilot frequencies in accordance with the invention has exactly the frequency required to eliminate all of the frequency error accumulated at previousrepeater points.

In completely eliminating accumulated frequency error on the final group dernodulator, the invention thus eliminates any need either for transmitting the group carrierV frequencies or for employing elaborate control ing group carrier frequency at all times and does notrequire manual adjustments. It provides,in short, the advantages of actual transmission of the carrier frequency without incurring the usual penalties.

In a number of important embodiment of the invention, the pilot frequencies which are used to derive the frequency deviation is the standard short-haul'A Bell System carrier telephone transmission system knownas type N carrier. This system, shown in outline form, for example, in United StatesPatent 2,695,332, which issued November 23, 1954,` to ,R. S. Caruthers, uses suppressedcarrier single-sideband techniques to transmit whole groups of double-sidebandV carrier-transmitted` channel signals. Repeaters are spaced periodically throughout the, system and, at each Vrepeater point, a Vlocal group carrier frequency is generatedfto modulate (or demodulate, depending upon the pointofview) the `group either from a low frequency Aband to a higher` frequency bandV or vice versa. Such alternation and inversion of channel groups between low` and highfrequency bands is final demodulating group carrier are harmonically related to one another when in either the high group or the low group, `in the sense that they are harmonics of a common fundamentalfrequency, and the received pilot frequencies are multiplied in frequency by the factors m and n in deriving the demodulating carrier, where m and n are integers greater than zero and satisfy the relation m=n+l The greatest simplicity in deriving the final group demodulating carrier is thereby assured.

A more complete understanding of the invention, along with other objects and features, may be obtained from a study of the following detailed description of several specific embodiments. In the drawings:

FIG. l is a block diagram of a typical N carrier system embodying` the invention arranged to transmit a group of twelve carrier-transmitted double-sideband channel signals;

FIGS. 2, 3, and 4 are blockY diagrams of alternative final lrepeatersforthe embodiment of the invention illustrated in FIG. l; and

FIG. 5 is a block diagram of a typical N carrier sys-l and V17 is eliminated.

tem embodying the invention arranged to transmit a single broadband signal channel.

FIG. l illustrates one direction of transmissionof a typical type N carrier'system Vmodified to operate in accordance with theV principles Yof the invention. The type N system operates'on a four-wire basisrequiring a simif lar arrangement for transmission in the reverse direction.

The circuitry required to provide the second path is not illustrated, however, since itY would merely be a substantial duplication of that shown.` In addition,'some filters, amplifiers, and other component circuits are not carrier system whenever one or more of the group carrier 4oscillators' at repeater points drifts from its assigned vnominal frequency of 304 kilocycles. As one of these carrieiz oscillators drifts, the sideband frequencies transmitted-from theassociated group modulator drifts in the same direction. `Successive carrier deviationsY along the repeatered line cause successive 'frequency errors in the transmitted-sidebands which, if they combine additively,

. can introduce a substantial frequency error in the group' separately illustrated where separateillustration is not necessary for an understanding of the invention. It isl to be understood, however, that in reality they `are includedV within appropriate ones of the blocks Y termed modulator, input circuit, and output circuit. Y

For normal telephone message transmission, the type Nis, as shown, a twelve channel system. VThe first channel includes, for example, Van appropriate input circuit'11 Vwhich supplies voice-frequency signalrwaves occupyingja band of approximately 300 to 3100 cycles to akchannel frequencies suppliedto thereceiving channel filters from the final repeater. This-frequency error tends to cause degradation in channel frequencyy characteristics Vbecause of misalignment ofthe incomingv frequency spectrum with modulator 12., Modulator 12 is vsupplied with its carrier i frequency'by a Vcarrier source 13. The other channels are similar, except for the channel carrier frequency em ployed, and will not be. separately described. n

Through the use of different carrier frequencies, the twelve channels are combined for transmission on a frequency-division multiplex basis, In the typervN system,Y

Vthe carriers are spaced at S-kilocycle intervals. These may be spaced, as illustrated,from 168 to 256 kilocycles,`

to forma so-called high group or from 48 to 136 'kilocycles to-form a so-called low group.V In each channel, the carrier and bothjupper and lower sidebandsare trans- .Y

mitted.

Repeaters in the type system employ s uppressedicar-`Y rier single-sideband techniques tofalternate the transmit-Y ted frequencies between a high band of from. v164Y to 12.60

kilocycles anda .low `band of `from '44 to 140 kilocycles at successive repeater points and invert-the frequency order atfeach point.YV As pointed out previously, this combination of frequency reversal Yand inversion is knownV Vas frogging'and serves to equalize noiseand crosstalk in the various signal channels. VTherepeatersy are'spaced at regular intervalsin the system, the particular spacing employed Vdepending principally upon cableV gauge. Each repeater normally includes a grouplmrodulator (so-called `rto-distinguish'it fromthe channel modulators), alocal source of carrier Vcurrent having a nominal 'frequency .of

304 kilocyclesQandtheappropriateamplifers 'and filters..

Threefrepeaters kare shown vin FIG.,`1,by`Way of example. i The first is shown comprising a group modulator Maud a localgnominal 3 04 kilocycle carriersourcelS andthe secondl is Vshowncomprising a group modulator 161and aV local y3704,'lrilocycleV carrier source Vi317.' As VexplainedV cycles in the low'group, these two carriers are in a-'2 to 1 harmonic relationship when in that band. A narrow band 192'ltilocycle `filter 22 is bridgedV across the lineV on the inputor high group side of group modulator 18 and a narrow band 56 kilocycle filterV V23 is similarly bridged across the line on the output or low group side. VBoth filters havepass Vbands wide enough to pass the chosen channel carriers even if shifted in frequency by the maximum anticipated lamount but'narrow Venough to exclude Y as many other frequencies as possible. Both filters'22 and 23 are connected to a local modulator 24, filter 22 directly and filter 23 through a frequency multiplyingV circuit 25.

Circuit 25xdoublesfthe frequencyY of the wave'appli'ed to it by flter23.V The sum modulation product of modulator 24Yis selected by a'304 kilocycle band-passfilter25 and amplified by an amplifier 27 before it is'v appliedY asv the Vdemodulating group vcarrier Y frequency to group modulator 18. Y v

The. effectiveness of the frequency; error 'cancellation Vafforded by the inventioncan be illustratedby a numerical example. The normal group demodulating'v wave frequency is304 kilocyclesiandtheselected channel carrier frequencies are 5`6 and 112 kilocycles. VSince-group modulator `18 isshown to be one .providing aftransition from the high band ,tok the low kband rather Ythan vice versa,

i these carriers on .the'riputsideof group. modulator'18 Y Ycycles and (192li-D).kilocycles, whereD is the` accumulated frequency deviation from V'the' preceding: repeaters.

The frequency (192+D) kilocycles is selectedY by filter 22 and applied to modulator .24. Ontheoutput side'of previously,eachy ,blockflabeledr fgroup modulator? YincludesrftheV appropriate filtersjand amplifiers forl'gsingle- -sideband carrier-suppressed transmission of the group frequencies. Ati the lastfrrepeaterpoint in FIG. ,1; theV- group Y modulator `V18 accomplishes, .theV 'final' `step of frequencyfrogging prior to .detectionfof the various channel'sig-V nals and, hence, can be.fsaidltofperformthe function'off. group Vdemodulation.y In accordance with the present invention, group modulator 18 has its demodulating'carrier o frequency derived in. such a mannerjtha'tlall frequency shift due. to inaccuracies of group carrier oscillators 15 The first type N carrierasignal channel in FIG, A1 is cornpleted, to the right of final group modulatoru 18,-by an appropriate VbandpassV chanfnelrfilter19,Y a channel degroup modulator, 18, the VVfrequency (i3-248.441)) kilo` cycles is selected by filter 23where P is the demodulating frequencyapplied togroupjmodulator 18 fromamplifier V27..V Modulatori24, combinesV these two Vfrequencies to'V yield the sum 'product (192+Dfl-2P-496-.2 D) Vkilocycles or (2P-.D.-304)'V kilocyeles. [Since this is the frequency selected by filterI 26andapplied as'the demodu- Y lating carrier to4 group modulator 1S, Y l P=2PD1f3ro4 A .Y

' Solved for P, thisequationyields A demodulating carrier thus` applied to groupmodulator Y Y Y 18 which includes the, precisefrequency deviation D that tector-j20, and a channel'outputl circuit21. The remain.-

'ing channels are completedA irra similarmanner, differing in the pass bands o f their chanel filters in order to` sepa! ratethe,variousgsign'al channels.` Sincethe Vcarrierand`V both sidebandsofthe signalfchannels Yare.received,:non

Alocal demodulating carrier source is required. .Y Y 1.

Unwanted:frequencyfdeviationfcreeps intoV a typejllV hasfbeen acquired bythe selected channelfcarrier'snand by all of the other transmitted' frequencies. In this ex latedby V group V"modulator n 18 asffollows: 5 `j s cervecera e156 'akiloadfa .Y 1 i v (Smink(19271Lmiizruqeyc1es Y ample, the two channel carriers `of interest aredemodu- All of the accumulated frequency deviation D has been` canceled in the final group modulator 18, whether it was l cycle or 100 cycles.

Although the selected low band channel carrier frequencies in the embodiment of the invention illustrated in FIG. l are 5,6 and 112 kilocycles, other combinations may be used as well. Examples are 40 and 80 kilocycles, 48 and 96 kilocycles, 64 and 128 kilocycles, and 52 and 104 kilocycles.

The particular arrangement illustrated in FIG. 1, in which the 192 kilocycle high band channel carrier is applied directly to modulator 24 is, of course, a special case. The 192 kilocycle wave is,vin a sense, multiplied in frequency by the factor l. A somewhat more general case is illustrated in FIG. 2, which shows a final type N carrier repeater embodying the invention in which a 208 kilocycle channel carrier in the high group is multiplied in frequency by 3 and an 80 kilocycle channel carrier in the low group is multiplied infrequency by 4. As illustrated, a 208 kilocycle bandpass filter 31 is bridged across the input side of final group modulator 1S and connected to modulator 24 through a frequency multiplying circuit 32 arranged to multiply the selected wave in frequency by the factor 3. VOn'the output side of group modulator 18, an 8O kilocycle band filter 33 is connected from the line to modulator 24 by a frequency multiplying circuit 34 arranged to multiply by the factor 4. The 304 kilocycle filter 26 connected to the output of modulator 24 selects the` difference-product to provide the correct demodulating frequency for canceling accumulated frequency error. V

It is, furthermore, not essential to the invention that the two channel carriers be selected from opposite sides of final group modulator 1S. FIGS. 3 and 4 illustrate embodiments of the invention Where both channel carriers are selected from the input side of group modulator 18. For purposes of illustration, the repeater in FIG. 3 is of the low-high typevwhile in FIG. 4 it is of the high-low type.

In the low-high repeater shown in FIG. 3, the selected channel carriers are in the 10W group and have normal frequencies of 64 and 104 kilocycles. A 64 kilocycle band filter 37 and a frequency multiplying circuit 38 for multiplying an incoming frequency by a factor of are connected in tandem between the line and modulator 24 Similarly connected are a 104 kilocycle band lter 39 and a frequency multiplying circuit 40 for multiplying the incoming frequency of a factor of 6. The output 304 kilocycle filter 26 selects the'difference frequency to provide a demodulating carrier which contains the frequency deviation of the two channel carriers and is exactly correct in frequency to cancel all accumulated Vfrequency error in the transmitted group signals. Y

The high-low repeater illustrated in FIG. 4 can be used as a basis for a large number of different channel carrier frequencies and frequency multiplying factor examples. Two channel carriers from the high group are used and the multiplying factors can be taken as mand n, which are any integers greater than Zero satisfying the relation hir-n+1 A first band filter 43 and frequency multiplier 44 combination is connected from the input side of group modulator 18 to one input of modulator 24. Frequency multiplier 44 is shown only in dashed outline form for the reason that the multiplying factor is unity in that path for the particular combination of channel carrier frequencies illustrated in FIG. 4. A second band filter 45 and freq quency multiplier 46 combination is connected from the same side of group modulator 18 to the other input of modulator 24. Y

If the high band channel carrier frequency selected by filter 45 is designated A, the frequency multiplying factor of frequency multiplier 46 is m, the high band channel carrier frequency selected by filter 43 is designated B,

and the factor of frequency multiplier 44 is n, a .number of effective combinations are shown in the following table:

A m B n i mA i nB Kc. Kc. Kc. t Kc. 240 3 208` 2 720 416 232 4 208 3 928 624 240 2 175 l 480 176 248 2 192 l V496 192 256 2 208 1 512 208 252` 2 200 1 504 200 The last four examples are particularly attractive because they require only a single frequency doubler in addition, to the filters and modulator 24 to generate the demodulating carrier.

While the basic embodiment of the invention illustrated in FIG. 1 makes use of the channel carriers already transmitted in the standard type N carrier telephone system, the principles underlying the invention are of sufficient breadth that their applicability is much greater. It is occassionally desirable, for example, to transmit a different type of signal over a repeatered type N carrier transmission line. FIG. 5 illustrates an embodiment 0f the invention in which the transmitted signal does not include channel carriers and in which a pair of pilot frequencies are used to generate the demodulating carrierwave instead. The transmitted signal may be, for example, a broadband data signal or a group of suppressed-carrier single sideband channels. As in FIG. l, only one direction of transmission is shown.

In FIG. 5, a signal input circuit 51 supplies a broadband signal occupying a band extending from 60 to 108 kilocycles to a repeatered N carrier line. The rst repeater contains a low-high group modulator 52 supplied with carrier yfrom a local nominal 304 kilocycle source 53. The second repeater contains a high-low group modulator 54 and a carrier source 55, while the third contains a low-high group modulator 56V supplied from a carrier source 57. A larger or smaller number of repeaters may be employed, of course, depending upon the length of the line and the gauge of the cable, but these are shown by Way of example. The final repeater contains a highlow group modulator 18 supplied with a demodulated carrier wave in accordance with the invention and furnishes the original 60 to 108 kilocycle signal to a signal output 58.

In accordance with the invention, a pair of pilot frequencies outside of the signal band but still within the pass band of the transmission system are introduced along with the signal band. In the embodiment illustrated in FIG. 5, the two pilot frequencies are 56 and 112 kilocycles. As illustrated, a 56 kilocycle source 59 is coupled to the line through a 56 kilocycle bandpass filter 60 and a 112 kilocycle source 61 is coupled to the line through a 112 kilocycle bandpass filter 62. These pilot frequencies are frogged up and down in frequency along with the main signal at each repeater point and accumulate the same frequency shift as the signal. At the final repeater, the demodulating carrier is supplied to group modulator 18 by a circuit combination substantially identical to the one shown in FIG. 1. The 56 kilocycle pilot frequency from the low group side of group modulator 18 is doubled infrequency and supplied to modulator 24 along with the 192 klocycle pilot from the high group side. The output filter 26 selects the sum product from modulator 24 and supplies it, through amplifier 27, to group modulator 18 as the demodulating carrier. As has already been explained, this carrier is precisely the frequency required to cancel all of the accumulated frequency error.

It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may bedevised by thoseskilled in thevartrwithout departingfrorn'the spirit and scope' of theinvention. u

What is claimed is:

1. In a suppressed-carrier amplitude'modulation signal transmission system havinga transmitting end and a receiving end, an Varrangement for'eliminating frequency error Whichrvcomprises means to Vtransmit apairof pilot frequencies ,over said system'along with the regular signal v from the-transmitting end to the receiving end, said pilot frequencies being harmonically related to one another, a

demodulator atthe receiving endV of said system, means to 'select said pilot frequencies at the receiving end of said system, means to multiply one of said pilot,frequencies in frequency by the factor m and the otherV by the factor n, Where m and Vn are integers greater than zero satisfying the relation m=nll, a second order modulator and a'means toderive a demodulating carrier for said demodulator by intermodulating said pilot frequencies quent to multiplication. 2 Y

' 2. In a suppressed-carrier amplitudemodulation system for transmittinga group Vof double-sideband carriertransmitted signals fromta transmitting station to a receiv ingstationj` arrangementrfor eliminating frequency error whichcomprises a demodulator at said receiving I u u 3,176,256A

Y Ywith one another in said second order modulator subsef station, Ymeansto select a pair of signal carrier frequencies Y at said receiving station, said selected signal carrier freqnencies being substantially harmonically related to one another, means to multiply one of said selected carrier frequencies in frequency/by the Vfactor m and the other by the. factor n, where inY and n 4are integers'. greater than zero satisfyingthe relation ml-.nll-l, a second order modulator, and Vmeans to derive apdemoclulatingV carrier for said demodulator by intermodulating said selected carrief `frequenciesrwith one anotherin said second order lmodulator to multiplication.

, 3. In a suppressed-carrier'amplitude modulation signal transmission system Vfor transmitting signals having a predetermined `frequency band from a transmitting station to Va receiving stationan arrangement for eliminating frequencyV error which comprises means to transmita pair of pilot frequencies outside of saidpredetermined frequency band but Within the pass band ofsaid system over said system along withV the regular V'signals from the transmitting'end tothe receiving end, a demodulator at Vs'aid receiving station, means Ito 'select said pilot frequencies at said receivingfstation, meansv tonmultiply oneV ofV said pilot frequenciesyin frequency by the factor m andthe Y other' by the factor 1,1,7Where m and n are'intergers greater than zero satisfying tht relation mf=nl1,V a' second order modulator, and means to derive a demodulating carrier vfor said Ydemoclulator by intermo'dulatinggsaid selected pilotl frequencies with one another in said secondorder modulator subsequent to multiplication.'

References Cited by the Examiner u UNITED STATES PATENTS 

1. IN A SUPPRESSED-CARRIER AMPLITUDE MODULATION SIGNAL TRANSMISSION SYSTEM HAVING A TRAINING END A RECEIVING END, AN ARRANGEMENT FOR ELIMINATING FRQUENY ERROR WHICH COMPRISES MEANS TO TRANSMIT A PAIR OF PILOT FREQUENCIES OVER SAID SYSTEM ALONG WITH THE REGULAR SIGNAL FROM THE TRANSMITTING END OF THE RECEIVING END, SAID PILOT FREQUENCIES BEING HARMONICALLY RELATED TO ONE ANOTHER, A DEMODULATOR AT THE RECEIVING END OF SAID SYSTEM, MEANS TO SELECT SAID PILOT FREQUENCIES AT THE RECEIVING END OF SAID SYSTEM, MEANS TO MULTIPLY ONE OF SAID PILOT FRE- 